(1) Field of the Invention
The present invention relates to a silicon nitride sintered body and a process for the preparation thereof. More particularly, the present invention relates to a silicon nitride sintered body excellent in the strength at high temperatures and the creep resistance, and a process for the preparation thereof.
(2) Description of the Prior Art
A sintered body composed of silicon nitride has a covalent bond as the main atom-linking bond and is excellent in strength, hardness and thermochemical stability, and therefore, this sintered body is used as engineering ceramics for a heat engine, for example, a gas turbine engine.
It is expected that with recent increase of the efficiency in a heat engine, the operation temperature of the heat engine will be elevated to a level higher than 1400.degree. C., and development of a matrial that can be used under this condition is desired.
As the sintering method for preparing silicon nitride, there have been adopted a hot press method and a pressureless sintering method, and a gas pressure sintering method is recently examined.
In these methods, it is intended to leave the added ingredients as a glass phase or crystal phase in the grain boundary or in the state solid-dissolved in the crystal phase of silicon nitride after sintering without decomposition.
If the additive ingredients are left in the sintered body, this results in suppression of excellent characteristics of silicon nitride owing to the covalent bond, such as a high strength at high temperatures and a high creep resistance. More specifically, in the case where an oxide type sintering aid is added, the sintering aid reacts with silicon nitride and a metal compound composed mainly of an oxide or nitride of silicon is formed in the grain boundary. This oxide or nitride promotes sintering and densification at the sintering step to improve the sintering property, but since this oxide or nitride has a strong ionic bonding property, the oxide or nitride tends to degrade the characteristics of the sintered body at high temperatures.
As means for controlling this tendency, there is proposed a method in which ZrO.sub.2 is used as the sintering aid to precipitate ZrO.sub.2 in the grain boundary of the sintered body and by utilizing the high melting point of ZrO.sub.2, the high-temperature characteristics of the sintered body are improved. However, if this sintered body is prepared according to any of the hot press method, the pressureless sintering method and the gas pressure sintering method, ZrO.sub.2 reacts with Si.sub.3 N.sub.4 to form ZrN in the sintered body. While this ZrN is converted to ZrO.sub.2 in a high-temperature oxidizing atmosphere, the volume is expanded by about 30%. Accordingly, the sintered body is readily cracked at the oxidation, and the ZrN-containing sintered body per se is very readily oxidized in a high-temperature oxidizing atmosphere and this sintered body is not satisfactory as a material to be used at high temperatures.